1. Field of the Invention
The present invention generally concerns measuring circuit characteristics of filters, and in more particular concerns built-in testing schemes for measuring the cut-off frequency of low-pass filters.
2. Background Information
Low pass filters are used in many applications where out of band components of a signal or noise have to be rejected. In many instances, they are implemented in mixed signal integrated circuits (ICs); such ICs are used extensively in telecommunications for pulse templating. For example, the transmitter in typical HDSL transceivers comprises a sigma delta digital-to-analog modulator followed by a large switched capacitor noise-shaping filter. In many cases, the bandwidth of the filter has to be measured to comply with template specifications. This requires the cut-off frequency of the filter to be measured.
Testing mixed signal ICs is known to be a difficult and time consuming task. A conventional test that is often used for testing the cut-off frequency of filters in mixed IC devices uses a multi-tone signal with a tone at the cut-off frequency and other tones at each side of cut-off frequency. During testing, the output signal of the device is digitized, and data processing is performed to extract the gains at each tone. In many cases, to ensure the compliance with different standards, the filter bandwidth of such ICs is programmable. Accordingly, when using the foregoing conventional testing method, a separate test must be performed for each bandwidth setting, leading to long production test times and expense. In addition, conventional test schemes often require connecting the circuit under test (CUT) to complex and expensive mixed-signal automated test equipment.
Ideally, it would be desirable to provide a test scheme for measuring the cut-off frequency of a low pass filter that is both fast and simple. Additionally, such a scheme should be implemented on-chip such that only a limited amount of automated testing equipment (ATE) is required, preferably requiring only digital ATE.